Many of the currently available passenger vehicles have automatically shifting transmissions to provide a plurality of discrete gear ratios between the engine and the drive wheels. Most of these transmissions employ at least one directional sensitive torque transmitting mechanism or one-way drive mechanism to accommodate a smooth ratio interchange. Generally this interchange occurs between the first forward ratio and the second forward ratio. The more premium of the automatic shifting transmissions utilize more than one direction sensitive torque transmitting mechanism.
These transmission mechanisms also have a positive torque transmitter in parallel with the direction sensitive mechanism to accommodate engine coast braking. For example, when negotiating a steep grade, it is desirable to manually engage one of the lower drive ratios such as low gear or second gear. If this ratio is established, during normal driving, by a one-way device, it becomes necessary to also include a selectively engageable friction torque transmitter, such as a clutch or brake, to enforce engine coast braking which will augment or perhaps supersede any necessary vehicle service brake application.
The direction sensitive torque transmitters most often found in the automotive vehicle transmission are of the roller or sprag variety. These one-way mechanisms of course add cost to the transmission. The friction torque transmitter used in parallel with these one-way devices is generally a multi-plate fluid operated mechanism. In many design configurations, the one-way device is located radially inward of a portion of the friction mechanism resulting in the friction mechanism having a larger outer diameter than necessitated by the design requirements.
In an effort to overcome some of the deficiencies of the combination of a one-way torque transmitting mechanism and a parallel friction torque transmitting mechanism, it has been proposed to employ a valve mechanism within the friction torque transmitter that is direction sensitive. One such device is described in a Society of Automotive Engineering (SAE) publication 930911 and U.S. Pat. Nos. 5,031,746 and 5,106,348 issued to Erkki Koivunen. While these devices are effective in eliminating the roller or sprag, they require additional valving and cam surfaces to be incorporated into the friction torque transmitting mechanism.